Google frowns at any form of paid link or unnatural linking, if you have been doing this it is better you stop before you get caught.

Copyright Violation

“Just because it is fine with Google doesn’t mean it is legal”.

Copyright violation is frowned upon by Google, hence you hear Google promoting the use of original content. Copyright violation is a surefire way of getting your pages de-indexed from Google

Masking Keywords

Hiding keywords or phrases with the background color technique is just so wrong and Google already knows about this technique. This could easily translate to your pages disappearing from Google search.

Google, Bing and every search engine hates spam. Always check pages you are linking to, even if it is a guest post, check the links to make sure you are not sending your readers to malicious pages.

Once you have been infected with malware or your site is linking to such a page, you are on a ticking clock.

These are just some of the ways you could get banned, blacklisted, de-indexed by Google. Always make sure you read and understand Google’s terms and keep yourself updated on their algorithm changes. Penguin, panda, hummingbird these are all algorithm changes your should read about as they could be responsible for your rapid drop in traffic.